Multi-piece solid golf ball

ABSTRACT

In a golf ball having a core, a cover and an intermediate layer therebetween, the ball and a sphere consisting of the core encased by the intermediate layer have surface hardnesses which satisfy a specific relationship, the intermediate layer and the cover have thicknesses which satisfy a specific relationship, and the core has a hardness profile in which the hardnesses at the core center, at positions 5 mm, 10 mm and 15 mm from the core center and at the core surface satisfy specific relationships. This ball, when played by mid- and high-level amateur golfers, achieves a good distance on driver shots, has a soft feel at impact and maintains the spin performance at a high level on approach shots.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2015-113941 filed in Japan on Jun. 4, 2015,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball of threeor more pieces having a core, an intermediate layer and a cover.

BACKGROUND ART

In the art relating to golf balls of two or more pieces having a coreand a cover and multi-piece solid golf balls of three or more pieceshaving a core, an intermediate layer and a cover, a number ofdisclosures have hitherto been made which focus on the hardness profilein the core or on the hardness relationship between the intermediatelayer and the cover, the intermediate layer material and the like. Suchgolf balls are described in, for example, US 2014-0187351 A1, JP-A2011-120898, JP-A 2010-214105, JP-A 2010-172702, JP-A 2008-194474 andJP-A 2008-194473.

However, there is room for further improvement in the core hardnessprofile of such golf balls. In particular, there exists a desire toprovide golf balls which, by optimizing the core hardness profile andthe overall hardness and thickness parameters of the ball, are able toachieve the intended spin properties and thus an increased distance.Moreover, in these golf balls, there is a desire not only for increaseddistance, but also, to increase the enjoyability of the game, for theability to maintain the spin performance on approach shots at a highlevel.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a multi-piecesolid golf ball which retains a good distance on shots with a driver(W#1) yet has a soft feel at impact and which, moreover, is able tomaintain the spin performance on approach shots at a high level.

As a result of extensive investigations, we have discovered that, in amulti-piece solid golf ball having a core, a cover and an intermediatelayer therebetween, by adjusting the design of the core hardness profileand hardness gradient such that the hardness gradient out to a position10 mm from the core center is not very steep but the hardness gradientfurther out from the core interior is steeper, with the hardnessdifference between a position 10 mm from the core center and the coresurface, expressed in terms of JIS-C hardness, being greater than 15,and moreover by constructing the ball such that the intermediate layeris thicker than the cover and the surface hardness of an intermediatelayer-encased sphere is higher than the surface hardness of the ball,the spin rate on full shots with a driver (W#1) can be held lower thanin conventional golf balls, enabling an increased distance to beachieved, in addition to which a soft feel at impact can be obtained.

Hence, we have succeeded in developing a superior golf ball which,particularly for the mid- or high-level amateur golfer whose head speedis not as high as that of a professional, retains the spin performanceon approach shots at a high level while maintaining a good distance onshots with a driver (W#1), and thus provides good enjoyability in thegame of golf. In addition, the golf ball of the invention also has anexcellent resistance to damage of the cover surface (scuff resistance)when struck with a fully grooved wedge. As used herein, “mid- andhigh-level amateur” refers to golfers having head speeds (HS) ofgenerally 40 to 50 m/s, with a mid-level amateur golfer having a HS ofabout 40 to 48 m/s and a high-level amateur golfer having a HS of about42 to 50 m/s.

Accordingly, the invention provides a multi-piece solid golf having acore, a cover and an intermediate layer therebetween, wherein a spheremade up of the core and the intermediate layer which peripherallyencases the core (intermediate layer-encased sphere) and the ball haverespective surface hardnesses, expressed in terms of Shore D hardness,which satisfy the relationship:

-   -   ball surface hardness≦surface hardness of intermediate        layer-encased sphere;        the intermediate layer and the cover have respective thicknesses        which satisfy the relationship:    -   cover thickness≦intermediate layer thickness; and        the core has a hardness profile which, expressed in terms of        JIS-C hardness, satisfies conditions (1) to (6) below, wherein        Cc is the JIS-C hardness at a center of the core, C5 is the        JIS-C hardness at a position 5 mm from the core center, C10 is        the JIS-C hardness at a position 10 mm from the core center, C15        is the JIS-C hardness at a position 15 mm from the core center,        and Cs is the JIS-C hardness at a surface of the core:

20≦Cs−Cc,  (1)

0<C10−Cc≦10,  (2)

C10−Cc<Cs−C10,  (3)

15<Cs−C10,  (4)

Cs≧80,  (5)

and

Cc≧52.  (6)

In a preferred embodiment, the golf ball further satisfies condition(3′) below:

(Cs−C10)/(C10−Cc)≧3.  (3′)

In another preferred embodiment, the golf ball further satisfiescondition (1′) below:

26≦Cs−Cc.  (1′)

In yet another preferred embodiment, the golf ball further satisfiescondition (7) below:

(C10−C5)≦(C5−C0)≦(Cs−C15)≦(C15−C10).  (7)

The core of the golf ball is preferably formed of a material moldedunder heat from a rubber composition containing: (A) a base rubber, (B)an organic peroxide, and (C) water and/or a metal monocarboxylate.

In the golf ball of the invention, letting tan δ₁ be the loss tangent ata dynamic strain of 1% and tan δ₁₀ be the loss tangent at a dynamicstrain of 10% when the loss tangents of the core center and the coresurface are measured at a temperature of −12° C. and a frequency of 15Hz, and defining the tan δ slope as (tan δ₁₀−tan δ₁)/(10%−1%), thedifference between the tan δ slope at the core surface and the tan δslope at the core center is preferably larger than 0.002.

The golf ball of the invention preferably satisfies the conditionV₀−V₆₀<0.7, where V₀ is the initial velocity of the core in the golfball after the intermediate layer and cover, collectively referred toherein as “the core-covering layers,” have been molded, as measuredafter peeling away the core-covering layers, and V₆₀ is the core initialvelocity measured 60 days after measuring V₀.

In the inventive golf ball, the intermediate layer is preferably formedof a resin composition containing: a combined amount of 100 parts byweight of the following two base resins (I) and (II):

-   -   (I) an olefin-unsaturated carboxylic acid-unsaturated carboxylic        acid ester terpolymer, or a metal neutralization product        thereof, having a weight-average molecular weight (Mw) of at        least 140,000, an acid content of 10 to 15 wt % and an ester        content of at least 15 wt %, and    -   (II) an olefin-acrylic acid binary random copolymer, or a metal        neutralization product thereof, having a weight-average        molecular weight (Mw) of at least 140,000 and an acid content of        10 to 15 wt %        blended in a weight ratio (I):(II) of from 90:10 to 10:90;    -   (III) from 1.0 to 2.5 parts by weight of a basic inorganic metal        compound capable of neutralizing un-neutralized acid groups in        the resin composition; and    -   (IV) from 1 to 100 parts by weight of an anionic surfactant        having a molecular weight of from 140 to 1500.        In this embodiment, the component (I) and (II) resins each have        a melt flow rate of 0.5 to 20 g/10 min, component (I) and        component (II) have a melt flow rate difference therebetween of        not more than 15 g/10 min, the composition of (I) to (IV) has a        melt flow rate of at least 1.0 g/10 min, and a molded material        obtained by molding the composition under applied heat has a        Shore D hardness of 35 to 60.

In a further embodiment, when the core surface is photographed with acamera and image data collected by the camera is image processed in suchmanner as to identify and digitize scratches appearing on the coresurface, the number of digitized scratches is 100 or more.

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention, when played by mid-and high-level amateur golfers, achieves a good distance on shots with adriver (W#1), has a soft feel at impact and maintains the spinperformance at a high level on approach shots, providing goodenjoyability in the game of golf. In addition, this golf ball has anexcellent resistance to damage of the cover surface (scuff resistance)when struck with a fully grooved wedge.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional diagram of a golf ball according to anembodiment of the invention.

FIG. 2 is a diagram illustrating a method of obtaining a test specimenfor measuring the bond strength between the core and the intermediatelayer of a golf ball.

FIG. 3 is a schematic diagram showing the apparatus used in the examplesto photograph a portion of the core surface.

FIG. 4 is an image showing a portion of a core surface that has beenphotographed by the apparatus in FIG. 3 and image-processed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become moreapparent from the following detailed description, taken in conjunctionwith the foregoing diagrams.

The multi-piece solid golf ball of the invention has, in order from theinside: a solid core, an intermediate layer, and a cover. Referring toFIG. 1, which shows the internal structure of one embodiment of theinventive golf ball, the golf ball G has a core 1, an intermediate layer2 encasing the core 1, and a cover 3 encasing the intermediate layer 2.Numerous dimples D are typically formed on the surface of the cover 3 toimprove the aerodynamic properties of the ball. The respective layersare described in detail below.

The core diameter, although not particularly limited, is generally from34.9 to 40.3 mm, preferably from 36.1 to 39.4 mm, and more preferablyfrom 37.3 to 38.5 mm. When the core diameter is too small, the spin rateon shots with a driver (W#1) may rise, as a result of which the intendeddistance may not be achieved. When the core diameter is too large, thedurability to cracking on repeated impact may worsen, or the feel of theball at impact may worsen.

The core deflection (mm) when compressed under a final load of 1,275 N(130 kgf) from an initial load of 98 N (10 kgf), although notparticularly limited, is preferably from 3.6 to 4.8 mm, more preferablyfrom 3.7 to 4.4 mm, and even more preferably from 3.8 to 4.2 mm. Whenthe core is too hard, the spin rate may rise excessively, resulting in apoor distance, and the feel of the ball may become too hard. On theother hand, when the core is too soft, the rebound may be too low,resulting in a poor distance, or the feel may become too soft and thedurability to cracking on repeated impact may worsen.

The core surface hardness (Cs), expressed in terms of JIS-C hardness, isat least 80, preferably from 80 to 90, more preferably from 81 to 88,and even more preferably from 82 to 86. When the JIS-C hardness valuefor the core surface hardness is too high, the feel of the ball may betoo hard or the durability to cracking on repeated impact may worsen. Onthe other hand, when this value is too low, the spin rate risesexcessively and the rebound is low, resulting in a poor distance.

The core center hardness (Cc), expressed in terms of JIS-C hardness, isat least 52, preferably from 52 to 63, more preferably from 53 to 61,and even more preferably from 55 to 59. When the JIS-C hardness valuefor the core center hardness is too high, the spin rate may riseexcessively, resulting in a poor distance, or the feel at impact may betoo hard. On the other hand, when this value is too low, the durabilityto cracking on repeated impact worsens and the feel at impact becomestoo soft.

The JIS-C hardness at a position 5 mm from the core center (C5) ispreferably from 54 to 66, more preferably from 56 to 64, and even morepreferably from 58 to 62. The JIS-C hardness at a position 10 mm fromthe core center (C10) is preferably from 55 to 68, more preferably from57 to 66, and even more preferably from 59 to 64. When these hardnessvalues are too high, the spin rate may rise excessively, resulting in apoor distance, or the feel at impact may be too hard. On the other hand,when these values are too low, the durability to cracking on repeatedimpact may worsen and the feel at impact may be too soft.

The above center hardness (Cc) and cross-sectional hardnesses atspecific positions refer to the hardnesses measured at the center andspecific positions on a cross-section obtained by cutting a golf ballcore in half through the center. The surface hardness (Cs) refers to thehardness measured on the spherical surface of the core.

The JIS-C hardness at a position 15 mm from the core center (C15) ispreferably from 68 to 82, more preferably from 70 to 80, and even morepreferably from 72 to 78. When this hardness value is too high, the feelat impact may become harder and the durability to cracking underrepeated impact may worsen. On the other hand, when this value is toolow, the spin rate may rise excessively and the rebound may decrease,resulting in a poor distance.

Next, in this invention, the core satisfies conditions (1) to (4) below:

20≦Cs−Cc,  (1)

0<C10−Cc≦10,  (2)

C10−Cc<Cs−C10,  (3)

and

15<Cs−C10  (4)

In condition (1), the value Cs−Cc is preferably from 21 to 32, morepreferably from 23 to 30, and even more preferably from 25 to 28. Whenthis value is too high, the durability to cracking on repeated impactworsens. On the other hand, when this value is too low, the spin raterises excessively and a good distance is not obtained.

In condition (2), it is critical for the value C10−Cc to be higher than0 and no higher than 10. This means that, in the core hardness profileof the invention, the gradient from the core center to a position 10 mmfrom the center is not very steep. The C10−Cc value is preferably from 1to 8, more preferably from 2 to 7, and even more preferably from 3 to 6.At a C10−Cc value outside of this range, the spin rate on full shotsrises and a good distance is not obtained, or the durability to crackingon repeated impact worsens.

In condition (3), it is critical for the value Cs−C10 to be higher thanthe value C10−Cc. This means that, in the core hardness profile of thisinvention, the gradient is steeper on the outside than at the coreinterior. In other words, the value (Cs−C10)/(C10−Cc) must be higherthan 1, and is preferably from 2 to 8, more preferably from 3 to 7, andeven more preferably from 4 to 6. When this value is too high, thedurability to cracking on repeated impact worsens. On the other hand,when this value is too low, the spin rate rises excessively and a gooddistance is not obtained.

In condition (4), it is critical for the value Cs−C10 to be at least 15.This means that, in the core hardness profile of this invention, thegradient from a position 10 mm from the core center (C10) to the coresurface (Cs) is steep to a degree that exceeds a JIS-C hardness of 15.The Cs−C10 value is preferably from 16 to 30, more preferably from 18 to28, and even more preferably from 20 to 26. When this value is toolarge, the durability to cracking on repeated impact may worsen. On theother hand, when this value is too small, the spin rate on full shotsmay rise, as a result of which a good distance may not be achieved.

In addition to above conditions (1) to (4), the core hardness profilemay be suitably adjusted so as to satisfy the following conditions.

The value Cs−C15, although not particularly limited, is preferably from3 to 14, more preferably from 5 to 12, and even more preferably from 7to 10. When this value is too high, the durability to cracking onrepeated impact may worsen. On the other hand, when this value is toolow, the spin rate may rise excessively, as a result of which a gooddistance may not be achieved.

The value C15−C10, although not particularly limited, is preferably from8 to 19, more preferably from 10 to 17, and even more preferably from 12to 15. When this value is too high, the durability to cracking onrepeated impact may worsen. On the other hand, when this value is toolow, the spin rate may rise excessively, as a result of which a gooddistance may not be achieved.

The value C10−C5, although not particularly limited, is preferably from−1 to 7, more preferably from 0 to 5, and even more preferably from 1 to3. When this value is outside of this range, the spin rate on full shotsmay rise excessively and a good distance may not be achieved, or thedurability to cracking on repeated impact may worsen.

The value C5−Cc, although not particularly limited, is preferably from 0to 8, more preferably from 1 to 6, and even more preferably from 2 to 4.When this value is too high, the spin rate may rise excessively and agood distance may not be achieved. On the other hand, when this value istoo low, the durability to cracking on repeated impact may worsen.

The core having the above hardness profile and deflection is preferablymade of a material that is composed primarily of rubber. For example,use may be made of a rubber composition obtained by compounding (A) abase rubber as the chief component, (B) an organic peroxide, and also aco-crosslinking agent, an inert filler and, optionally, an organosulfurcompound.

Polybutadiene is preferably used as the base rubber (A). Thepolybutadiene has a cis-1,4 bond content on the polymer chain oftypically at least 60 wt %, preferably at least 80 wt %, more preferablyat least 90 wt %, and most preferably at least 95 wt %. When the contentof cis-1,4 bonds among the bonds on the polybutadiene molecule is toolow, the resilience may decrease.

Rubber components other than this polybutadiene may be included in thebase rubber (A) within a range that does not detract from theadvantageous effects of the invention. Examples of such rubbercomponents other than the foregoing polybutadiene include otherpolybutadienes, and diene rubbers other than polybutadiene, such asstyrene-butadiene rubber, natural rubber, isoprene rubber andethylene-propylene-diene rubber.

The organic peroxide (B) used in the invention is not particularlylimited, although the use of an organic peroxide having a one-minutehalf-life temperature of 110 to 185° C. is preferred. One, two or moreorganic peroxides may be used. The amount of organic peroxide includedper 100 parts by weight of the base rubber is preferably at least 0.1part by weight, and more preferably at least 0.3 part by weight. Theupper limit is preferably not more than 5 parts by weight, morepreferably not more than 4 parts by weight, and even more preferably notmore than 3 parts by weight. A commercially available product may beused as the organic peroxide. Specific examples include those availableunder the trade names Percumyl D, Perhexa C-40, Niper BW and Peroyl L(all from NOF Corporation), and Luperco 231XL (from Atochem Co.).

The co-crosslinking agent is exemplified by unsaturated carboxylic acidsand the metal salts of unsaturated carboxylic acids. Illustrativeexamples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred. Metal salts of unsaturatedcarboxylic acids are not particularly limited, and are exemplified bythose obtained by neutralizing the foregoing unsaturated carboxylicacids with the desired metal ions. Illustrative examples include thezinc salts and magnesium salts of methacrylic acid and acrylic acid. Theuse of zinc acrylate is especially preferred.

These unsaturated carboxylic acids and/or metal salts thereof areincluded in an amount per 100 parts by weight of the base rubber whichis typically at least 10 parts by weight, preferably at least 15 partsby weight, and more preferably at least 20 parts by weight. The upperlimit is typically not more than 60 parts by weight, preferably not morethan 50 parts by weight, more preferably not more than 45 parts byweight, and most preferably not more than 40 parts by weight. When toomuch is included, the feel of the ball may become too hard andunpleasant. When too little is included, the rebound may decrease.

To satisfy the desired hardness profile described above, the core ispreferably formed of a material molded under heat from a rubbercomposition which includes, as the essential ingredients: (A) a baserubber, (B) an organic peroxide, and (C) water and/or a metalmonocarboxylate.

Decomposition of the organic peroxide within the core formulation can bepromoted by the direct addition of water (or a water-containingmaterial) to the core material. It is known that the decompositionefficiency of the organic peroxide within the core-forming rubbercomposition changes with temperature and that, starting at a giventemperature, the decomposition efficiency rises with increasingtemperature. If the temperature is too high, the amount of decomposedradicals rises excessively, leading to recombination between radicalsand, ultimately, deactivation. As a result, fewer radicals acteffectively in crosslinking. Here, when a heat of decomposition isgenerated by decomposition of the organic peroxide at the time of corevulcanization, the vicinity of the core surface remains at substantiallythe same temperature as the temperature of the vulcanization mold, butthe temperature near the core center, due to the build-up of heat ofdecomposition by the organic peroxide which has decomposed from theoutside, becomes considerably higher than the mold temperature. In caseswhere water (or a water-containing material) is added directly to thecore, because the water acts to promote decomposition of the organicperoxide, radical reactions like those described above can be made todiffer at the core center and at the core surface. That is,decomposition of the organic peroxide is further promoted near thecenter of the core, bringing about greater radical deactivation, whichleads to a further decrease in the amount of active radicals. As aresult, it is possible to obtain a core in which the crosslink densitiesat the core center and the core surface differ markedly. It is alsopossible to obtain a core having different dynamic viscoelasticproperties at the core center. Along with achieving a lower spin rate,golf balls having such a core are also able to exhibit excellentdurability and undergo less change over time in rebound. When zincmonoacrylate is used instead of the above water, water is generated fromthe zinc monoacrylate by heat during kneading of the compoundingmaterials. An effect similar to that obtained by the addition of watercan thereby be obtained.

Components A and B have already been described above.

The water serving as component C is not particularly limited, and may bedistilled water or tap water. The use of distilled water which is freeof impurities is especially preferred. The amount of water included per100 parts by weight of the base rubber is preferably at least 0.1 partby weight, and more preferably at least 0.3 part by weight. The upperlimit is preferably not more than 5 parts by weight, and more preferablynot more than 4 parts by weight.

By including a suitable amount of such water, the moisture content inthe rubber composition prior to vulcanization becomes preferably atleast 1,000 ppm, and more preferably at least 1,500 ppm. The upper limitis preferably not more than 8,500 ppm, and more preferably not more than8,000 ppm. When the moisture content of the rubber composition is toolow, it may be difficult to obtain a suitable crosslink density and tanδ, which may make it difficult to mold a golf ball having little energyloss and a reduced spin rate. On the other hand, when the moisturecontent of the rubber composition is too high, the core may end up toosoft, which may make it difficult to obtain a suitable core initialvelocity.

It is also possible to add water directly to the rubber composition. Thefollowing methods (i) to (iii) may be employed to include water:

-   (i) applying steam or ultrasonically applying water in the form of a    mist to some or all of the rubber composition (compounded material);-   (ii) immersing some or all of the rubber composition in water;-   (iii) letting some or all of the rubber composition stand for a    given period of time in a high-humidity environment in a place where    the humidity can be controlled, such as a constant humidity chamber.

As used herein, “high-humidity environment” is not particularly limited,so long as it is an environment capable of moistening the rubbercomposition, although a humidity of from 40 to 100% is preferred.

Alternatively, the water may be worked into a jelly state and added tothe above rubber composition. Or a material obtained by first supportingwater on a filler, unvulcanized rubber, rubber powder or the like may beadded to the rubber composition. In such a form, the workability isbetter than when water is added directly to the composition, enablingthe efficiency of golf ball production to be increased. The type ofmaterial in which a given amount of water has been included, althoughnot particularly limited, is exemplified by fillers, unvulcanizedrubbers and rubber powders in which sufficient water has been included.The use of a material which causes no loss of durability or resilienceis especially preferred. The moisture content of the above material ispreferably at least 3 wt %, more preferably at least 5 wt %, and evenmore preferably at least 10 wt %. The upper limit is preferably not morethan 99 wt %, and even more preferably not more than 95 wt %.

In this invention, a metal monocarboxylate may be used instead of theabove-described water. Metal monocarboxylates, in which the carboxylicacid is presumably coordination-bonded to the metal, are distinct frommetal dicarboxylates such as zinc diacrylate of the formula(CH₂═CHCOO)₂Zn. A metal monocarboxylate introduces water into the rubbercomposition by way of a dehydration/condensation reaction, and thusprovides an effect similar to that of water. Moreover, because a metalmonocarboxylate can be added to the rubber composition as a powder, theoperations can be simplified and uniform dispersion within the rubbercomposition is easy. A monosalt is required in order to carry out theabove reaction effectively. The amount of metal monocarboxylate includedper 100 parts by weight of the base rubber is preferably at least 1 partby weight, and more preferably at least 3 parts by weight. The upperlimit in the amount of metal monocarboxylate included is preferably notmore than 60 parts by weight, and more preferably not more than 50 partsby weight. When the amount of metal monocarboxylate included is toosmall, it may be difficult to obtain a suitable crosslink density andtan δ, as a result of which a sufficient golf ball spin rate-loweringeffect may not be achievable. On the other hand, when too much isincluded, the core may become too hard, as a result of which it may bedifficult for the ball to maintain a suitable feel at impact.

The carboxylic acid used may be, for example, acrylic acid, methacrylicacid, maleic acid, fumaric acid or stearic acid. Examples of thesubstituting metal include sodium, potassium, lithium, zinc, copper,magnesium, calcium, cobalt, nickel and lead, although the use of zinc ispreferred. Illustrative examples of the metal monocarboxylate includezinc monoacrylate and zinc monomethacrylate, with the use of zincmonoacrylate being especially preferred.

Core production may be carried out in the usual manner by molding aspherical molded article (core) using heat and compression undervulcanization conditions of at least 140° C. and not more than 180° C.and at least 10 minutes and not more than 60 minutes.

The vulcanized core preferably has a higher moisture content at the corecenter than at the core surface. The moisture content of the molded corecan be suitably controlled by adjusting such conditions as the amount ofwater included in the rubber composition, the molding temperature andthe molding time.

The moisture content at the core center, although not particularlylimited, is preferably at least 1,000 ppm, more preferably at least1,200 ppm, and even more preferably at least 1,500 ppm. The upper limitis preferably not more than 7,000 ppm, more preferably not more than6,000 ppm, and even more preferably not more than 5,000 ppm. Themoisture content at the core surface, although not particularly limited,is preferably at least 800 ppm, more preferably at least 1,000 ppm, andeven more preferably at least 1,200 ppm. The upper limit is preferablynot more than 5,000 ppm, more preferably not more than 4,000 ppm, andeven more preferably not more than 3,000 ppm. The (moisture content atcore surface)−(moisture content at core center) value is preferably 0ppm or below, more preferably −100 ppm or below, and even morepreferably −200 ppm or below. The lower limit value is preferably −1,000ppm or above, more preferably −700 ppm or above, and even morepreferably −600 ppm or above.

Measurement of the above moisture content may be carried out withordinary instruments. For example, the moisture content can be measuredusing the AQ-2100 coulometric Karl Fischer titrator and the EV-2000evaporator (both available from Hiranuma Sangyo Co. Ltd.) at ameasurement temperature of 130° C., a preheating time of 3 minutes and abackground measurement time of 30 seconds.

Letting V₀ be the initial velocity of the core measured after removingthe intermediate layer and cover (which layers are referred to hereincollectively as the “core-covering layers”) from a ball obtained bymolding these core-covering layers over the core and V₆₀ be the initialvelocity of the core measured 60 days after the day on which V₀ wasmeasured, V₀ is preferably at least 77.0 m/s, more preferably at least77.1 m/s, and even more preferably at least 77.2 m/s, but is preferablynot more than 78.5 m/s, more preferably not more than 78.3 m/s, and evenmore preferably not more than 78.0 m/s. V₆₀ is preferably at least 77.0m/s, and more preferably at least 77.1 m/s, but is preferably not morethan 77.8 m/s, more preferably not more than 77.7 m/s, and even morepreferably not more than 77.6 m/s. When core initial velocities V₀ andV₆₀ within the above ranges cannot be obtained, achieving a satisfactorydistance is difficult. Also, if the core initial velocity is too high,the golf ball may not conform to the Rules of Golf. Because thecore-covering layer materials are not readily permeable to moisture inthe atmosphere, there are cases where the change in core initialvelocity over time cannot be measured using the ball as is or where ittakes a long time for such change to occur. Therefore, by removing thecore-covering layers and exposing the core itself to the atmosphere, itis possible to reliably measure the change in core initial velocity overtime.

The value V₀−V₆₀ preferably satisfies the relationship V₀−V₆₀<0.7, morepreferably satisfies the relationship V₀−V₆₀<0.6, and still morepreferably satisfies the relationship V₀−V₆₀<0.5. In this invention,when moisture has been included in a good balance within the core, evenif the core comes into direct contact with the atmosphere, it is notreadily influenced by the atmospheric humidity, enabling changes in thecore initial velocity to be suppressed.

In this invention, the core initial velocity may be measured using aninitial velocity measuring apparatus of the same type as the USGA drumrotation-type initial velocity instrument approved by the R&A. The coremay be tested for this purpose in a chamber at a room temperature of23±2° C. after being held isothermally in a 23±1° C. environment for atleast 3 hours.

Next, the method of measuring the dynamic viscoelasticity of the core isexplained. In this invention, letting tan δ₁ be the loss tangent at adynamic strain of 1% and tan δ₁₀ be the loss tangent at a dynamic strainof 10% when the loss tangents are measured at a temperature of −12° C.and a frequency of 15 Hz in a dynamic viscoelasticity test on vulcanizedrubber at the core center and core surface, and defining the tan δ slopeas (tan δ₁₀−tan δ₁)/(10%−1%), a desirable feature of the invention isthat the difference between the tan δ slope at the core surface and thetan δ slope at the core center be larger than 0.002. This difference inslopes is preferably larger than 0.003, and more preferably larger than0.004. At a smaller difference in slope, the energy loss by the coreends up being larger, making a spin rate-lowering effect more difficultto obtain. Various methods may be employed to measure the dynamicviscoelastic properties of the core. For example, a circular disk havinga thickness of 2 mm may be cut out of the cover-encased core by passingthrough the geometric center thereof and then, treating this disk as thesample, using a punching machine to punch out 3 mm diameter specimens atthe places of measurement. In addition, by employing a dynamicviscoelasticity measuring apparatus (such as that available under theproduct name EPLEXOR 500N from GABO) and using a compression testholder, the tan δ values under dynamic strains of 0.01 to 10% can bemeasured at an initial strain of 35%, a measurement temperature of −12°C. and a frequency of 15 Hz, and the slopes determined based on theresults of these measurements.

Regarding the viscoelastic behavior measured in this way, there is knownto be a correlation between the viscoelastic behavior in the high-strainregion and the spin rate of the golf ball when struck. Thus, when thetan δ in the high-strain region is relatively large, i.e., when the tanδ slope between a dynamic strain of 10% and a dynamic strain of 1% islarge, the spin rate rises; conversely, when the tan δ in thehigh-strain region is relatively small, i.e., when the tan δ slopebetween a dynamic strain of 10% and a dynamic strain of 1% is small, thespin rate falls. Also, the amount of deformation varies depending on theclub used to strike the golf ball, with deformation occurring even atthe ball center when the ball is struck with a driver or a middle iron(e.g., a number six iron). Therefore, when striving to reduce the spinrate on shots with a driver or a number six iron, good results can beobtained by making the tan δ slope between a dynamic strain of 10% and adynamic strain of 1% at the core center small. In cases where thedeformation on impact is small, such as on approach shots near thegreen, the influence of the tan δ at the core surface is large. Hence,to increase or maintain the spin rate on approach shots, good resultscan be obtained by making the tan δ slope between a dynamic strain of10% and a dynamic strain of 1% at the core surface large. Accordingly,to obtain a golf ball that travels well on shots with a driver and stopson approach shots, what is desired is for the tan δ slope between adynamic strain of 10% and a dynamic strain of 1% at the core center tobe made small and for the tan δ slope between a dynamic strain of 10%and a dynamic strain of 1% at the core surface to be made large; thatis, for the difference between the tan δ slope at the core surface andthe tan δ slope at the core center to be made large.

In this invention, when the core surface is photographed with a cameraand image data collected by the camera is image processed in such manneras to identify and digitize scratches appearing on the core surface, thenumber of digitized scratches is preferably 100 or more, and morepreferably 200 or more. By expressing the degree of core surfaceroughness in terms of the number of scratches, an attempt is made hereto evaluate adhesion between the core and the intermediate layerencasing the core. This is based on the observation that the number ofscratches on the core surface has an influence on adhesion between thecore and the intermediate layer. In a core having a small number of suchscratches, the adhesive strength between the core and the intermediatelayer is weak, likely resulting in an inadequate durability to crackingby the core itself.

The method of measuring scratches on the core surface is based on thetechnology of capturing a photographic image for the purpose ofdigitizing the roughness of the core surface as the number of scratches,and processing the captured image. Use may be made of, for example, theimage processing technique and equipment described below.

The measuring equipment setup may include, for example, as shown in FIG.3, a stand 50 on which the object to be measured (i.e., the core) isplaced, lighting means 60 situated above the stand 50, and a camera 70situated even higher. FIG. 3 also shows a power supply 90 for thelighting means 60. Individual elements of the setup are arrangedrelative to one another in a structure that can be finely adjusted toprovide clear photographic image data for the core being measured.Enclosing the outer periphery of the equipment with a blackout curtainor the like is desirable for minimizing the influence of the outsideenvironment. The image data captured with the lighting means 60 andcamera 70 is imported to a computer 100, where it is image processed anddigitized using specific image processing software that has beeninstalled in the computer. Various commercially available products maybe used as the camera, lighting means, computer, image processingsoftware and the like in the setup shown in FIG. 3. The stand 50 onwhich the core is placed and the frame 80 for securing the camera,lighting, etc. are ordinary structures for which detailed descriptionsare omitted here.

Using the image processing software, processing is carried out which,basically, treats regions of the captured image data that are darkerthan a threshold setting for brightness and where there are at least aset number of connected pixels (connectivity number) in such dark areas(shadows) as scratches, and counts the number of such scratches. Thebrightness threshold and the connectivity number setting used forcounting the number of scratches are made to agree with the depth andsize of scratches generally visible on the core surface. The depth andsize of generally visible scratches are each about 0.5 mm, althoughthese thresholds and settings may be adjusted as necessary. It is alsopossible to classify scratches by the degree of darkness, to classifythe size of scratches by the size of regions of connected shadows or,from the shape characteristics of connected shadows, to classify theshape of scratches by prioritizing shapes that are linear, for example.When carrying out such shape processing, because the photographed imagesurface of the core surface is curved rather than flat, it is preferableto use a dynamic threshold method.

Specific commercially available products that may be used as the imageprocessing equipment and image software (image processing means) arediscussed in the subsequently described examples. However, because ofrapid development recently in image processing equipment, the equipmentmentioned in the examples may not necessarily be the most suitable.Therefore, it is advisable to select appropriate equipment in keepingwith the core to be photographed and recent technical advances, and tocombine and use such equipment together.

In this invention, as mentioned above, by specifying the roughness ofthe core surface in terms of a desired number of scratches, the adhesivestrength between the core and the intermediate layer described below canbe increased. Specifically, based on the method of measuring adhesivestrength described in the examples, the adhesion may be set topreferably at least 1.00 N/4 mm, and more preferably at least 1.15 N/4mm. In order to achieve a core surface having such a high adhesion, thecore material and intermediate layer material may be suitably selectedand a method for abrading the core surface may be employed. Abrasion ofthe core surface may be carried out using a known method and under knownconditions.

Next, the intermediate layer is described. The intermediate layer has amaterial hardness expressed in terms of Shore D hardness which, althoughnot particularly limited, is preferably from 48 to 68, more preferablyfrom 52 to 62, and even more preferably from 55 to 57. The sphereencased by the intermediate layer (referred to below as the“intermediate layer-encased sphere”) has a surface hardness, expressedin terms of Shore D hardness, which is preferably from 55 to 75, morepreferably from 59 to 69, and even more preferably from 62 to 64. Whenthe intermediate layer is too soft, the spin rate on full shots may riseexcessively, as a result of which a good distance may not be achieved.On the other hand, when the intermediate layer is too hard, thedurability to cracking on repeated impact may worsen and the feel of theball on shots with a putter or on short approaches may worsen.

The intermediate layer-encased sphere has a deflection (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) which, although not particularly limited, is preferablyfrom 2.8 to 4.0 mm, more preferably from 3.0 to 3.8 mm, and even morepreferably from 3.2 to 3.6 mm. When this value is too high, the feel ofthe ball may be too soft, the durability to repeated impact may be poor,or the initial velocity on full shots may be low, as a result of whichthe intended distance may not be achieved. On the other hand, when thisvalue is too low, the feel of the ball may be too hard and the spin rateon full shots may rise, as a result of which the intended distance maynot be achieved.

The intermediate layer has a thickness of preferably from 0.9 to 2.4 mm,more preferably from 1.2 to 2.1 mm, and even more preferably from 1.5 to1.8 mm. In this invention, it is preferable for the thickness of theintermediate layer to be higher than that of the subsequently describedcover (outermost layer). When the intermediate layer thickness isoutside of this range or thinner than the cover, the spin rate-reducingeffects on driver (W#1) shots may be inadequate, as a result of which agood distance may not be achieved.

The intermediate layer material is not particularly limited, althoughpreferred use can be made of various thermoplastic resin materials. Fromthe standpoint of fully achieving the desired effects of the invention,it is especially preferable to use a high-resilience resin material asthe intermediate layer material. For example, the use of an ionomerresin material or the subsequently described highly neutralized resinmaterial is preferred.

Specifically, a molded material obtained by molding a resin compositionof components (I) to (IV) described below under applied heat may be usedas the highly neutralized resin material.

Preferred use can be made of the two following components (I) and (II)as the base resins:

-   (I) An olefin-unsaturated carboxylic acid-unsaturated carboxylic    acid ester terpolymer, or a metal neutralization product thereof,    having a weight-average molecular weight (Mw) of at least 140,000,    an acid content of 10 to 15 wt % and an ester content of at least 15    wt %; and-   (II) An olefin-acrylic acid random copolymer, or a metal    neutralization product thereof, having a weight-average molecular    weight (Mw) of at least 140,000 and an acid content of 10 to 15 wt    %.

The weight-average molecular weight (Mw) of component (I) is at least140,000, and preferably at least 145,000. The weight-average molecularweight (Mw) of component (II) is at least 140,000, and preferably atleast 160,000. By thus making these molecular weights large, the resinmaterial can be assured of having sufficient resilience.

It is thought that because the acid components and ester contents of therespective copolymers serving as the base resins (I) and (II) differ,these two types of base resins interlock in a complex manner, givingrise to molecular synergistic effects that can increase the rebound anddurability of the ball. In this invention, by specifying theweight-average molecular weight, acid content and ester content asindicated above in such a way as to select a material that is relativelysoft as the terpolymer serving as base resin (I), and by specifying thetype of acid, weight-average molecular weight and acid content in such away as to select a relatively hard material as base resin (II), it ispossible with a blend of these polymers to ensure sufficient resilienceand durability for use as a golf ball material.

Here, the weight-average molecular weight (Mw) is a value calculatedrelative to polystyrene in gel permeation chromatography (GPC). A wordof explanation is needed here concerning GPC molecular weightmeasurement. It is not possible to directly take GPC measurements forcopolymers and terpolymers because these molecules are adsorbed to theGPC column owing to unsaturated carboxylic acid groups within themolecules. Instead, the unsaturated carboxylic acid groups are generallyconverted to esters, following which GPC measurement is carried out andthe polystyrene-equivalent average molecular weights Mw and Mn arecalculated.

The olefins used in component (I) and component (II) preferably have 2to 6 carbons, with ethylene being especially preferred. The unsaturatedcarboxylic acid used in component (I) is not particularly limited,although preferred use can be made of acrylic acid or methacrylic acid.To ensure resilience, the unsaturated carboxylic acid used in component(II) is acrylic acid. This is because, when methacrylic acid is used asthe unsaturated carboxylic acid in component (II), the methacrylic acidwith its pendant methyl group may give rise to a buffering action,lowering the reactivity.

The unsaturated carboxylic acid content (acid content) within each ofcomponents (I) and (II), although not particularly limited, ispreferably at least 10 wt %, with the upper limit being preferably lessthan 15 wt %, and more preferably less than 13 wt %. When this acidcontent is low, moldings of the golf ball material may lack sufficientresilience. On the other hand, when the acid content is high, thehardness may become excessively high, adversely affecting thedurability.

The unsaturated carboxylic acid ester used in the terpolymer serving ascomponent (I) is preferably a lower alkyl ester, with butyl acrylate(butyl n-acrylate, butyl i-acrylate) being especially preferred.

The ester content of the unsaturated carboxylic acid ester in component(I), in order to employ a resin that is relatively soft compared withthe binary copolymer serving as component (II), is set to at least 15 wt%, preferably at least 18 wt %, and more preferably at least 20 wt %,with the upper limit being preferably not more than 25 wt %. At an estercontent higher than this range, moldings of the intermediate layermaterial may lack sufficient resilience. On the other hand, when theester content is low, the hardness may increase, adversely affecting thedurability.

The hardness of the base resin (I), that is, the hardness when the resinitself is molded alone (material hardness), expressed in terms of ShoreD hardness, is preferably at least 30, and more preferably at least 35,with the upper limit being preferably not more than 50, and morepreferably not more than 45. The hardness of the base resin (II), thatis, the hardness when the resin itself is molded alone (materialhardness), expressed in terms of Shore D hardness, is preferably atleast 40, and more preferably at least 50, with the upper limit beingpreferably not more than 60, and more preferably not more than 57. Whenbase resins outside of these respective hardness ranges are used, amaterial having the desired hardness may not be obtained, or an adequateresilience and durability may not be obtained.

In this invention, it is preferable for component (I) and component (II)to be used together. The mixing proportions of component (I) andcomponent (II), expressed as the weight ratio (I):(II), is set topreferably 90:10 to 10:90, more preferably 85:15 to 30:70, and even morepreferably 80:20 to 50:50. When the proportion of component II is higherthan this range, the hardness increases, as a result of which thematerial may be difficult to mold.

When metal neutralization products of resins (i.e., ionomers) are usedas component (I) and component (II), the type of metal neutralizationproduct and the degree of neutralization are not particularly limited.Illustrative examples include 60 mol % Zn (degree of neutralization withzinc) ethylene-methacrylic acid copolymers, 40 mol % Mg (degree ofneutralization with magnesium) ethylene-methacrylic acid copolymers, and40 mol % Mg (degree of neutralization with magnesium)ethylene-methacrylic acid-acrylic acid ester terpolymers.

To ensure at least a given degree of flowability during injectionmolding and provide a good molding processability, it is essential forthe melt flow rates of the resins serving as components (I) and (II) toeach be from 0.5 to 20 g/10 min. The difference between the melt flowrates of components (I) and (II) is set to not more than 15 g/10 min.When the difference in melt flow rates between these base resins is toolarge, the components cannot be uniformly mixed together during thecompounding of components (I) and (II) in an extruder, and so themixture becomes non-uniform, which may lead to injection moldingdefects.

As noted above, copolymers or ionomers with weight-average molecularweights (Mw) set in specific ranges are used as components (I) and (II).Illustrative examples of commercial products that may be used for thispurpose include the Nucrel series (DuPont-Mitsui Polychemicals Co.,Ltd.), the Escor series (ExxonMobil Chemical), the Surlyn series (E.I.DuPont de Nemours & Co.), and the Himilan series (DuPont-MitsuiPolychemicals Co., Ltd.).

In addition, (III) a basic inorganic metal compound is preferablyincluded as a component for neutralizing acid groups in above components(I) and (II) and subsequently described component (IV). By even morehighly neutralizing the resin material in this way, the spin rate of theball on full shots is even further reduced without adversely affectingthe feel of the ball, thus making an increased distance fully achievableeven by amateur golfers. Illustrative examples of the metal ions in thebasic inorganic metal compound include Na⁺, K⁺, Li⁺, Zn²⁺, Ca²⁺, Me, Ceand Co²⁺. Of these, Na⁺, Zn²⁺, Ca²⁺ and Mg²⁺ are preferred, and Mg²⁺ ismore preferred. These metal salts may be introduced into the resinusing, for example, formates, acetates, nitrates, carbonates,bicarbonates, oxides and hydroxides.

This basic inorganic metal compound (III) is included in the resincomposition in an amount equivalent to at least 70 mol %, based on theacid groups in the resin composition. Here, the amount in which thebasic inorganic metal compound serving as component (III) is includedmay be selected as appropriate for obtaining the desired degree ofneutralization. Although this amount depends also on the degree ofneutralization of the base resins (components (I) and (II)) that areused, in general it is preferably from 1.0 to 2.5 parts by weight, morepreferably from 1.1 to 2.3 parts by weight, and even more preferablyfrom 1.2 to 2.0 parts by weight, per 100 parts by weight of the combinedamount of the base resins (components (I) and (II)). The degree ofneutralization of the acid groups in components (I) to (IV) ispreferably at least 70 mol %, more preferably at least 90 mol %, andeven more preferably at least 100 mol %.

Next, the anionic surfactant serving as component (IV) is described. Thereason for including an anionic surfactant is to improve the durabilityafter resin molding while ensuring good flowability of the overall resincomposition. The anionic surfactant is not particularly limited,although the use of one having a molecular weight of from 140 to 1,500is preferred. Exemplary anionic surfactants include carboxylatesurfactants, sulfonate surfactants, sulfate ester surfactants andphosphate ester surfactants. Preferred examples include one, two or moreselected from the group consisting of various fatty acids such asstearic acid, behenic acid, oleic acid and maleic acid, derivatives ofthese fatty acids, and metal salts thereof. Selection from the groupconsisting of stearic acid, oleic acid and mixtures thereof isespecially preferred. Alternatively, exemplary organic acid metal saltsthat may serve as component (IV) include metal soaps, with the metalsalt being one in which a metal ion having a valence of 1 to 3 is used.The metal is preferably selected from the group consisting of lithium,sodium, magnesium, aluminum, potassium, calcium and zinc, with the useof metal salts of stearic acid being especially preferred. Specifically,the use of magnesium stearate, calcium stearate, zinc stearate or sodiumstearate is preferred.

Component (IV) is included in an amount, per 100 parts by weight of thebase resins serving as components (I) and (II), of 1 to 100 parts byweight, preferably 10 to 90 parts by weight, and more preferably 20 to80 parts by weight. When the component (IV) content is too low, it maybe difficult to lower the hardness of the resin material. On the otherhand, at a high content, the resin material is difficult to mold andbleeding at the material surface increases, adversely affecting themolded article.

In this invention, the moldability of the material and the productivitycan be further increased by suitably adjusting the compounding ratiobetween components (III) and (IV). When the content of the basicinorganic metal compound serving as component (III) is too high, theamount of gases such as organic acids that evolve during moldingdecreases, but the flowability of the material diminishes. Conversely,when the content of component (III) is low, the amount of gasesgenerated increases. On the other hand, when the content of the anionicsurfactant serving as component (IV) is too high, the amount of gasconsisting of fatty acids and other organic acids increases duringmolding, which has a large impact in terms of molding defects andproductivity. Conversely, when the content of component (IV) is low, theamount of gases generated decreases, but the flowability and durabilitydecline. Therefore, achieving a proper compounding balance betweencomponents (III) and (IV) is also important. Specifically, it isdesirable to set the compounding ratio between components (III) and(IV), expressed as the weight ratio (III):(IV), to from 4.0:96.0 to1.0:99.0, and especially from 3.0:97.0 to 1.5:98.5.

The resin composition of above components (I) to (IV) accounts forpreferably at least 50 wt %, more preferably at least 60 wt %, even morepreferably at least 70 wt %, and most preferably at least 90 wt %, ofthe total amount of the intermediate layer material.

A non-ionomeric thermoplastic elastomer may be included in theintermediate layer material. The non-ionomeric thermoplastic elastomeris preferably included in an amount of from 1 to 50 parts by weight per100 parts by weight of the combined amount of the base resins.

The non-ionomeric thermoplastic elastomer is exemplified by polyolefinelastomers (including polyolefins and metallocene-catalyzedpolyolefins), polystyrene elastomers, diene polymers, polyacrylatepolymers, polyamide elastomers, polyurethane elastomers, polyesterelastomers and polyacetals.

Optional additives may be suitably included in the intermediate layermaterial according to the intended use. For example, various additivessuch as pigments, dispersants, antioxidants, ultraviolet absorbers andlight stabilizers may be added. When such additives are included, thecontent thereof per 100 parts by weight of components (I) to (IV)combined is preferably at least 0.1 part by weight, and more preferablyat least 0.5 part by weight, with the upper limit being preferably notmore than 10 parts by weight, and more preferably not more than 4 partsby weight.

It is advantageous to abrade the surface of the intermediate layer inorder to increase adhesion with the polyurethane that is preferably usedin the subsequently described cover (outermost layer). In addition, itis desirable to apply a primer (adhesive) to the surface of theintermediate layer following such abrasion treatment or to add anadhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which istypically less than 1.1, preferably from 0.90 to 1.05, and morepreferably from 0.93 to 0.99. Outside of this range, the rebound becomessmall, as a result of which a good distance may not be obtained, or thedurability to cracking on repeated impact may worsen.

Next, the cover, which is the outermost layer of the ball, is described.

The cover (outermost layer) has a material hardness expressed in termsof Shore D hardness which, although not particularly limited, ispreferably from 44 to 58, more preferably from 48 to 56, and even morepreferably from 52 to 54.

The cover (outermost layer) encased sphere, i.e., the ball, has asurface hardness expressed in terms of Shore D hardness which ispreferably from 52 to 67, more preferably from 56 to 65, and even morepreferably from 60 to 63. When the cover-encased sphere is too muchsofter than this range, the spin rate on driver (W#1) shots and ironshots may become too high, as a result of which a good distance may notbe obtained. When the cover is too much harder than this range, the spinrate on approach shots may be inadequate or the feel at impact may betoo hard.

The cover (outermost layer) encased sphere, that is, the ball, has adeflection (mm) when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf) which, although not particularlylimited, is preferably from 2.4 to 3.7 mm, more preferably from 2.6 to3.5 mm, and even more preferably from 2.8 to 3.3 mm. When this value istoo high, the feel of the ball may be too soft, the durability torepeated impact may worsen, or the initial velocity on full shots may below, as a result of which the intended distance may not be achieved. Onthe other hand, when this value is too low, the feel of the ball may betoo hard and the spin rate on full shots may rise, as a result of whichthe intended distance may not be achieved.

The cover (outermost layer) has a thickness which, although notparticularly limited, is preferably from 0.3 to 1.5 mm, more preferablyfrom 0.45 to 1.2 mm, and even more preferably from 0.6 to 0.9 mm. Whenthe cover is thicker than this range, the rebound on W#1 shots and ironshots may be inadequate and the spin rate may rise, as a result of whicha good distance may not be obtained. On the other hand, when the coveris thinner than this range, the scuff resistance may worsen and the ballmay lack spin receptivity on approach shots, resulting in poorcontrollability.

The cover (outermost layer) material is not particularly limited,although the use of any of various types of thermoplastic resinmaterials is preferred. For reasons having to do with controllabilityand scuff resistance, it is preferable to use a urethane resin as thecover material of the invention. In particular, from the standpoint ofthe mass productivity of manufactured golf balls, it is preferable touse a cover material composed primarily of a thermoplastic polyurethane,with formation more preferably being carried out using a resin blendcomposed primarily of (O) a thermoplastic polyurethane and (P) apolyisocyanate compound.

In the thermoplastic polyurethane composition containing abovecomponents (O) and (P), to improve the ball properties even further, anecessary and sufficient amount of unreacted isocyanate groups should bepresent in the cover resin material. Specifically, it is recommendedthat the combined weight of above components (O) and (P) be at least60%, and more preferably at least 70%, of the weight of the overallcover layer. Components (O) and (P) are described below in detail.

The thermoplastic polyurethane (O) has a structure which includes softsegments composed of a polymeric polyol (polymeric glycol) that is along-chain polyol, and hard segments composed of a chain extender and apolyisocyanate compound. Here, the long-chain polyol serving as astarting material may be any that has hitherto been used in the artrelating to thermoplastic polyurethanes, and is not particularlylimited. Illustrative examples include polyester polyols, polyetherpolyols, polycarbonate polyols, polyester polycarbonate polyols,polyolefin polyols, conjugated diene polymer-based polyols, castoroil-based polyols, silicone-based polyols and vinyl polymer-basedpolyols. These long-chain polyols may be used singly, or two or more maybe used in combination. Of these, in terms of being able to synthesize athermoplastic polyurethane having a high rebound resilience andexcellent low-temperature properties, a polyether polyol is preferred.

Any chain extender that has hitherto been employed in the art relatingto thermoplastic polyurethanes may be advantageously used as the chainextender. For example, low-molecular-weight compounds with a molecularweight of 400 or less which have on the molecule two or more activehydrogen atoms capable of reacting with isocyanate groups are preferred.Illustrative, non-limiting, examples of the chain extender include1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanedioland 2,2-dimethyl-1,3-propanediol. Of these, an aliphatic diol having 2to 12 carbons is preferred, and 1,4-butylene glycol is more preferred,as the chain extender.

Any polyisocyanate compound hitherto employed in the art relating tothermoplastic polyurethanes may be advantageously used withoutparticular limitation as the polyisocyanate compound. For example, usemay be made of one, two or more selected from the group consisting of4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. However, depending on the typeof isocyanate, the crosslinking reaction during injection molding may bedifficult to control. In the practice of the invention, to provide abalance between stability at the time of production and the propertiesthat are manifested, it is most preferable to use the following aromaticdiisocyanate: 4,4′-diphenylmethane diisocyanate.

Commercially available products may be used as the thermoplasticpolyurethane serving as component (O). Illustrative examples includePandex T-8295, T-8290, T-8283 and T-8260 (all from DIC Bayer Polymer,Ltd.).

Although not an essential ingredient, a thermoplastic elastomer otherthan the above thermoplastic polyurethane may be included as anadditional component together with above components (O) and (P). Byincluding this component (Q) in the above resin blend, a furtherimprovement in the flowability of the resin blend can be achieved andthe properties required of a golf ball cover material, such asresilience and scuff resistance, can be enhanced.

The relative proportions of above components (O), (P) and (Q) are notparticularly limited. However, to fully elicit the desirable effects ofthe invention, the weight ratio (O):(P):(Q) is preferably from 100:2:50to 100:50:0, and more preferably from 100:2:50 to 100:30:8.

In addition to the ingredients making up the thermoplastic polyurethane,various additives may be optionally included in the above resin blend.For example, pigments, dispersants, antioxidants, light stabilizers,ultraviolet absorbers and internal mold lubricants may be suitablyincluded.

The manufacture of multi-piece solid golf balls in which theabove-described core, intermediate layer and cover (outermost layer) areformed as successive layers may be carried out by a customary methodsuch as a known injection-molding process. For example, a multi-piecegolf ball may be obtained by placing a molded and vulcanized productcomposed primarily of a rubber material as the core in a given injectionmold, injecting an intermediate layer material over the core to give anintermediate sphere, and subsequently placing the resulting sphere inanother injection mold and injection-molding a cover (outermost layer)material over the sphere. Alternatively, a cover may be formed over theintermediate layer by a method that involves encasing the intermediatesphere with a cover (outermost layer), this being carried out by, forexample, enclosing the intermediate sphere within two half-cups thathave been pre-molded into hemispherical shapes, and then molding underapplied heat and pressure.

The golf ball of the invention preferably satisfies the followingconditions.

(1) Relationship Between Deflections of Core and Ball Under SpecificLoading

The relationship between the deflections of the core and the ball underspecific loading is optimized within a specific range. That is, lettingCH be the deflection of the core when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and BH be thedeflection of the ball when compressed under a final load of 1,275 N(130 kgf) from an initial load of 98 N (10 kgf), the value CH−BH ispreferably from 0.7 to 1.5, more preferably from 0.9 to 1.3, and evenmore preferably from 1.0 to 1.2. When this value is too large, thedurability to cracking on repeated impact may worsen, or the feel of theball on full shots may be too soft. On the other hand, when this valueis too small, the spin rate on full shots may become too high, as aresult of which the intended distance may not be obtained.

(2) Relationship Between Thicknesses of Intermediate Layer and Cover

The relative thicknesses of the intermediate layer and the cover are setin a specific range. The value obtained by subtracting the coverthickness from the intermediate layer thickness is preferably from 0 to2.0 mm, more preferably from 0.1 to 1.5 mm, and even more preferablyfrom 0.3 to 1.0 mm. When this value is too large, the feel at impact maybecome too hard or the core may become too soft, resulting in a poordurability to cracking on repeated impact. On the other hand, when thisvalue is too small, the spin rate on full shots may become too high, asa result of which the intended distance may not be obtained.

(3) Relationship Between Surface Hardnesses of Ball and IntermediateLayer-Encased Sphere

In order for the ball to have a structure in which the cover is hard onthe inside and soft on the outside and the intermediate layer is hard,it is critical for the surface hardnesses of the ball and theintermediate layer-encased sphere to satisfy the relationship:

-   -   surface hardness of ball≦surface hardness of intermediate        layer-encased sphere.        The value obtained by subtracting the surface hardness of the        intermediate layer-encased sphere from the surface hardness of        the ball, expressed in terms of Shore D hardness, is preferably        from −20 to 0, more preferably from −15 to −1, and even more        preferably from −10 to −2. When this value is too large, the        spin rate on full shots may rise excessively, as a result of        which the intended distance may not be obtained, or the cover        may become hard, giving the ball an inadequate spin rate in the        short game, as a result of which the controllability may be        poor. On the other hand, when this value is too small, the cover        may become too soft, leading to excessive spin on full shots, or        the initial velocity may be too low, as a result of which the        intended distance may not be achieved.

(4) Relationship Between Surface Hardnesses of Core and Ball

The relationship between the surface hardnesses of the core and the ballis optimized within a specific range. That is, the value obtained bysubtracting the surface hardness of the ball from the surface hardnessof the core, expressed in terms of Shore D hardness, is preferably from−15 to 5, more preferably from −8 to −4, and even more preferably from−7 to −5. When this value is too large, the cover may be too hard,making the ball poorly suited for the short game, or the core may besoft, which may result in a poor durability to cracking on repeatedimpact. On the other hand, when this value is too small, the spin rateon full shots may rise excessively, as a result of which the intendeddistance may not be obtained.

(5) Relationship Between Deflections Under Specific Loading of Core andIntermediate Layer-Encased Sphere

Letting CH be the deflection of the core when compressed under a finalload of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and MHbe the deflection of the intermediate layer-encased sphere whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), the value CH−MH is preferably from 0.3 to 1.4, morepreferably from 0.5 to 1.2, and even more preferably from 0.7 to 1.0.When this value is too large, the durability to cracking on repeatedimpact may worsen, or the initial velocity of the ball on full shots maydecrease, as a result of which the intended distance may not beobtained. On the other hand, when this value is too small, the spin rateon full shots may become too high, as a result of which the intendeddistance may not be obtained.

(6) Relationship Between Surface Hardnesses of IntermediateLayer-Encased Sphere and Core

The relationship between the surface hardnesses of the intermediatelayer-encased sphere and the core is optimized within a specific range.That is, the value obtained by subtracting the surface hardness of thecore from the surface hardness of the intermediate layer-encased sphere,expressed in terms of Shore D hardness, is preferably from 3 to 20, morepreferably from 5 to 15, and even more preferably from 7 to 10. Whenthis value is too large, the durability to cracking under repeatedimpact may worsen, or the feel at impact may worsen. On the other hand,when this value is too small, the spin rate on full shots may be toohigh, as a result of which the intended distance may not be obtained.

Numerous dimples may be formed on the cover (outermost layer). Thenumber of dimples arranged on the cover surface, although notparticularly limited, is preferably at least 280, more preferably atleast 300, and even more preferably at least 320, with the upper limitbeing preferably not more than 360, more preferably not more than 350,and even more preferably not more than 340. When the number of dimplesis larger than this range, the ball trajectory becomes lower, as aresult of which the distance may decrease. On the other hand, when thenumber of dimples is too small, the ball trajectory becomes higher, as aresult of which a good distance may not be achieved.

The dimple shapes that are used may be of one type or a combination oftwo or more types selected from among circular shapes, various polygonalshapes, dewdrop shapes and oval shapes. When circular dimples are used,the dimple diameter may be set to at least about 2.5 mm and up to about6.5 mm, and the dimple depth may be set to at least 0.08 mm and up toabout 0.30 mm.

In order to fully manifest the aerodynamic properties, it is desirablefor the surface coverage ratio of dimples on the spherical surface ofthe golf ball, i.e., the ratio SR of the sum of the individual dimplesurface areas, each defined by the flat plane circumscribed by the edgeof a dimple, with respect to the spherical surface area of the ball wereit to have no dimples thereon, to be set to at least 60% and up to 90%.Also, to optimize the ball trajectory, it is desirable for the valueV_(o), defined as the spatial volume of the individual dimples below theflat plane circumscribed by the dimple edge, divided by the volume ofthe cylinder whose base is the flat plane and whose height is themaximum depth of the dimple from the base, to be set to at least 0.35and up to 0.80. Moreover, it is preferable for the ratio VR of the sumof the spatial volumes of the individual dimples, each formed below theflat plane circumscribed by the edge of a dimple, with respect to thevolume of the ball sphere were the ball surface to have no dimplesthereon, to be set to at least 0.6% and up to 1.0%. Outside of the aboveranges in these respective values, the resulting trajectory may notenable a good distance to be obtained, and so the ball may fail totravel a fully satisfactory distance.

The multi-piece solid golf ball of the invention can be made to conformto the Rules of Golf for play. Specifically, the inventive ball may beformed to a diameter which is such that the ball does not pass through aring having an inner diameter of 42.672 mm and is not more than 42.80mm, and to a weight which is preferably from 45.0 to 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided toillustrate the invention, and are not intended to limit the scopethereof.

Examples 1 and 2, Comparative Examples 1 to 7 Formation of Core

Solid cores for the respective Examples of the invention and ComparativeExamples were produced by preparing the rubber compositions shown inTable 1 below, then molding and vulcanizing the compositions under thevulcanization conditions shown in the same table.

TABLE 1 Core formulations Example Comparative Example (pbw) 1 2 1 2 3 45 6 7 Polybutadiene A 80 80 80 80 80 80 80 80 80 Polybutadiene B 20 2020 20 20 20 20 20 20 Zinc acrylate 37.0 34.3 28.5 25.5 23.0 34.3 37.028.5 28.5 Organic peroxide (1) 1.0 1.0 1.0 1.0 Organic peroxide (2) 2.52.5 2.5 2.5 2.5 Water 0.8 0.8 0.05 0.05 0.05 0.8 0.8 0.05 0.05Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Barium sulfate (1) 15.716.8 16.8 11.6 Barium sulfate (2) 18.2 19.5 20.6 18.2 18.2 Zinc oxide4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 Zinc salt of 0.6 0.6 0.4 0.4 0.4 0.60.6 0.4 0.4 pentachlorothiophenol Vulcanization Temp. 155 155 155 155155 155 155 155 155 (° C.) conditions Time 15 15 15 15 15 15 15 15 15(min)

Details on the ingredients shown in Table 1 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR    Corporation-   Polybutadiene B: Available under the trade name “BR 51” from JSR    Corporation-   Zinc acrylate: Available from Nihon Jyoryu Co., Ltd.-   Organic peroxide (1): Dicumyl peroxide, available under the trade    name “Percumyl D” from NOF Corporation-   Organic peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane    and silica, available under the trade name “Perhexa C-40” from NOF    Corporation-   Water: Distilled water, from Wako Pure Chemical Industries, Ltd.-   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available    under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical    Industry Co., Ltd.-   Barium sulfate (1): Available under the trade name “Barico #300”    from Hakusui Tech-   Barium sulfate (2): Available as “Precipitated Barium Sulfate #100”    from Sakai Chemical Co., Ltd.-   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from    Sakai Chemical Co., Ltd.-   Zinc stearate: Available under the trade name “Zinc Stearate G” from    NOF Corporation-   Sulfur: Available under the trade name “Sulfax-5” from Tsurumi    Chemical Industry Co., Ltd.

Formation of Intermediate Layer and Cover

An intermediate layer material formulated as shown in Table 2 wasinjected-molded over the core obtained above, thereby giving anintermediate layer-encased sphere. Next, using the cover materialsformulated as shown in Table 2, a cover (outermost layer) wasinjection-molded over the resulting intermediate layer-encased sphere,thereby producing a golf ball having an intermediate layer and a cover(outermost layer) over the core. Although not shown in the diagram, acommon dimple pattern was formed on the surface of the ball in each ofthe Examples of the invention and the Comparative Examples.

TABLE 2 Resin materials (pbw) I II III T-8295 75 100 T-8290 25 Surlyn9320 10 AN 4221C 90 Hytrel 4001 11 11 Titanium oxide 3.9 3.9Polyethylene wax 1.2 1.2 Isocyanate compound 7.5 7.5 Magnesium stearate60 Magnesium oxide 2.1 Polytail H 8

Details on the materials shown in Table 2 are as follows.

-   T-8295, T-8290: MDI-PTMG type thermoplastic polyurethanes available    from DIC Bayer Polymer under the trademark Pandex.-   Surlyn 9320: An ethylene-methacrylic acid-acrylic acid ester    terpolymer available from E.I. DuPont de Nemours & Co.-   AN 4221C: An unneutralized ethylene-acrylic acid copolymer available    from DuPont-Mitsui Polychemicals Co., Ltd.-   Hytrel 4001: A polyester elastomer available from DuPont-Toray Co.,    Ltd.-   Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical    Industries, Ltd.-   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate-   Magnesium oxide: “Kyowamag MF 150” from Kyowa Chemical Industry Co.,    Ltd.-   Polytail H: Available from Mitsubishi Chemical Corporation

For each of the resulting golf balls, properties such as the thicknessesand material hardnesses of the respective layers and the surfacehardnesses of various layer-encased spheres were evaluated by themethods described below. The results are shown in Table 3. In addition,the flight performance, properties on approach shots, feel, and scuffresistance for each golf ball were evaluated as described below. Thoseresults are shown in Table 4.

Core Hardness Profile

The indenter of a durometer was set so as to be substantiallyperpendicular to the spherical surface of the core, and the core surfacehardness in terms of JIS-C hardness was measured as specified in JISK6301-1975.

To obtain the cross-sectional hardnesses at the center and otherspecific positions of the core, the core was hemispherically cut so asform a planar cross-section, and measurements were carried out bypressing the indenter of a durometer perpendicularly against thecross-section at the measurement positions. These hardnesses areindicated as JIS-C hardness values.

The Shore D hardness at the core surface was measured with a type Ddurometer in accordance with ASTM D2240-95.

Dynamic Viscoelastic Properties of Core

A circular disk having a thickness of 2 mm was cut out by passingthrough the geometric center of the core and, treating the core centerand surface vicinity on this disk as the respective samples, a punchingmachine was used to punch out 3 mm diameter specimens at the places ofmeasurement. The loss tangents (tan δ) under dynamic strains of from0.01% to 10% were measured at an initial strain of 35%, a measurementtemperature of −12° C. and a frequency of 15 Hz using a dynamicviscoelasticity measuring apparatus (such as that available under theproduct name EPLEXOR 500N from GABO) and a compression test holder.Measurement results obtained within a radius of 5 mm from the corecenter were treated as the tan δ at the core center, and measurementresults within 5 mm of the core surface were treated as the tan δ at thecore surface.

Core Moisture Content

Using the AQ-2100 coulometric Karl Fischer titrator and the EV-2000evaporator (both available from Hiranuma Sangyo Co., Ltd.), measurementof the moisture content was carried out at a measurement temperature of130° C., a preheating time of 3 minutes and a background measurementtime of 30 seconds. The interval time was set to 99 seconds and thecurrent was set to “Fast.” Measurement results obtained within a radiusof 5 mm from the core center were treated as the moisture content forthe center of the core, and measurement results obtained within 5 mm ofthe core surface were treated as the moisture content for the surface ofthe core.

Initial Velocity of Core after Standing

A core was prepared by peeling the intermediate layer and cover from agolf ball. The core initial velocity measured on the day that thecore-covering layers—these being the intermediate layer and cover—werepeeled off was treated as the Day 0 result, and the initial corevelocity when 60 days had elapsed thereafter was treated as the Day 60result. During this time, the core was kept in a chamber controlled to atemperature of 24° C. and 40% humidity. The initial velocity wasmeasured using an initial velocity measuring apparatus of the same typeas the USGA drum rotation-type initial velocity instrument approved bythe R&A. The core was tested in a chamber at a room temperature of 23±2°C. after being held isothermally in a 23±1° C. environment for at least3 hours. Twenty cores were each hit twice. The time taken for the coreto traverse a distance of 6.28 ft (1.91 m) was measured and used tocompute the initial velocity. This cycle was carried out over a periodof about 15 minutes.

Core Surface Roughness

A grinding wheel was mounted on a centerless grinder commonly used forgrinding spheres and the core surface was abraded for 5 seconds at 2,500rpm, following which the surface roughness of the core was determined bythe following method. An electrodeposited diamond wheel (40/50 grit) wasused as the grinding wheel in Examples 1 and 2 of the invention and inComparative Examples 1 to 5, and a common grinding wheel (GC 46)differing in the frequency of use was used in Comparative Examples 6 and7. Core surface data was collected from an area having a diameter ofabout 10 mm at each of five places and the data for each subjected toimage processing, thereby obtaining the number of scratches on the coresurface. Next, using the average value for the five places as themeasured value for a single ball, the average value for five balls (N=5)was determined. The measurement apparatus and method were as shown inFIG. 3 and described above. The following commercial equipment was usedin the apparatus shown in FIG. 3.

Lighting means 60: UV LED lamp (LDR2-60VL385-BTTPTK), from CCS Inc.Lighting power source 90: PD3-3024-3-PI, also from CCS Inc.Camera 70: Sony XC-73 CCD cameraComputer 100: A PC using Windows™ 7 as the operating system, and HALCONsold by LinX Corporation as the image processing softwareFIG. 4 is an image showing part of the core surface obtained by imageprocessing with the above image processing software. Non-black regionsare shadows; because these are regions darker than a threshold setting,they basically indicate scratches. In the processing carried out in thepractice of the invention, regions of 30 or more connected pixels werecounted as scratches; the number of scratches in this image isconsidered to be 95. In FIG. 4, regions that appear as dots do notsatisfy the connectivity number setting (here, an unbroken sequence of30 pixels), and so are not included in the number of scratches.

Bond Strength (Peel Value) Between Core and Intermediate Layer

Referring to FIG. 2, in a sphere composed of a core 1 encased by anintermediate layer 2, two parallel cuts 11, 12 spaced 4.0 mm apart weremade in the intermediate layer 2 in such a way as to pass entirelythrough this layer, and the intermediate layer 2 at both ends of thesphere was peeled off. Next, a lateral cut 13 that passes entirelythrough the intermediate layer 2 was made at a right angle to the firsttwo cuts 11, 12, after which the bond strength was measured byimmobilizing the core portion 1 and pulling on the cut end of theintermediate layer 2. Measurement was carried out using an Instrontester and based on JIS K6256 (“Adhesion Test Method for VulcanizedRubber and Thermoplastic Rubber”). Using the specially prepared testspecimen described above, the clamp was moved at a speed of 50 mm/minand the tensile strength was measured at 0.1 mm intervals. The averageof the tensile strengths for three test specimens, after discarding thefirst quarter and the last quarter of all the measurement points, wastreated as the bond strength (units: N).

Diameter of Core or Intermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at atemperature of 23.9±1° C. and, using the average of these measurementsas the measured value for a single core or intermediate layer-encasedsphere, the average diameter for five measured cores or intermediatelayer-encased spheres was determined.

Ball Diameter

The diameters at five random dimple-free areas on the surface of a ballwere measured at a temperature of 23.9±1° C. and, using the average ofthese measurements as the measured value for a single ball, the averagediameter for five measured balls was determined.

Deflection of Core, Intermediate Layer-Encased Sphere and Ball

A core, intermediate layer-encased sphere or ball was placed on a hardplate and the amount of deflection when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured.The amount of deflection here refers in each case to the measured valueobtained after holding the test specimen isothermally at 23.9° C. In thetable, letting A be the core deflection, B be the deflection by theintermediate layer-encased sphere and C be the ball deflection, thevalues A-B and A-C were calculated.

Material Hardnesses of Intermediate Layer and Cover (Shore D Hardnesses)

The intermediate layer and cover-forming resin materials were moldedinto sheets having a thickness of 2 mm and left to stand for at leasttwo weeks, following which the Shore D hardnesses were measured inaccordance with ASTM D2240-95.

Surface Hardnesses of Intermediate Layer-Encased Sphere and Ball (ShoreD Hardnesses)

Measurements were taken by pressing the durometer indenterperpendicularly against the surface of the intermediate layer-encasedsphere or ball (i.e., the surface of the cover). The surface hardness ofthe ball (cover) is the measured value obtained at dimple-free places(lands) on the ball surface. The Shore D hardnesses were measured with atype D durometer in accordance with ASTM D2240-95.

TABLE 3 Example Comparative Example 1 2 1 2 3 Structure 3-piece 3-piece3-piece 3-piece 3-piece Core Diameter (mm) 37.7 37.7 37.7 37.7 37.7Weight (g) 32.9 32.9 32.7 32.7 32.7 Deflection A (mm) 3.9 4.2 3.8 4.24.8 Hardness profile (JIS-C) Surface hardness (Cs) 85 82 82 79 75Hardness at position 76 73 72 69 66 15 mm from center (C15) Hardness atposition 62 60 69 65 61 10 mm from center (C10) Hardness at position 6159 69 65 61 5 mm from center (C5) Center hardness (Cc) 57 56 61 59 56 Cs− C15 8 9 9 10 9 C15 − C10 14 13 3 4 4 C10 − C5 2 1 0 0 0 C5 − C0 3 3 86 5 Cs − C10 22 22 13 13 13 C10 − Cc 5 4 8 6 5 (Cs − C10)/(C10 − Cc) 4.45.5 1.6 2.1 2.6 Surface − Center (Cs − Cc) 27 26 21 20 19 Surfacehardness (Shore D) 56 54 54 52 49 tan δ at 0.1% strain 0.0420 0.04000.0440 0.0420 0.0420 core center 1% strain 0.0450 0.0440 0.0460 0.04300.0460 10% strain 0.0580 0.0550 0.1000 0.1070 0.1050 tan δ slope for0.0014 0.0012 0.0060 0.0071 0.0066 10% strain and 1% strain tan δ at0.1% strain 0.0730 0.0690 0.0750 0.0750 0.0770 core surface 1% strain0.0750 0.0720 0.0790 0.0780 0.0800 10% strain 0.1320 0.1350 0.14000.1480 0.1440 tan δ slope for 0.0063 0.0070 0.0068 0.0078 0.0071 10%strain and 1% strain Difference in tan δ slopes 0.0049 0.0058 0.00080.0007 0.0006 Core Center (ppm) 2020 1980 992 1059 1017 moisture Surface(ppm) 1802 1827 1795 1845 1833 content Surface − Center (ppm) −218 −153803 786 816 Initial Day 0 of standing (V0), m/s 77.69 77.45 77.63 77.3677.18 velocity of Day 60 of standing (V60), m/s 77.29 77.03 76.88 76.6476.47 core after Initial velocity 0.40 0.42 0.75 0.72 0.71 standingdifference (V0 − V60), m/s Core surface Number of scratches 220 245 280305 315 roughness Core and Peel value (N/4 mm) 1.12 1.18 1.31 1.32 1.34intermediate layer Intermediate Material I I I I I layer Thickness (mm)1.7 1.7 1.7 1.7 1.7 Specific gravity 0.95 0.95 0.95 0.95 0.95 Materialhardness (Shore D) 56 56 56 56 56 Intermediate Diameter (mm) 41.1 41.141.1 41.1 41.1 layer-encased Weight (g) 40.8 40.8 40.6 40.6 40.6 sphereDeflection B (mm) 3.3 3.5 3.3 3.7 4.1 Surface hardness (Shore D) 63 6363 63 63 Intermediate layer surface hardness − 7 9 9 11 14 Core surfacehardness (Shore D) Deflection difference (A − B) 0.7 0.7 0.5 0.5 0.7Cover Material II II II II II Thickness (mm) 0.8 0.8 0.8 0.8 0.8Specific gravity 1.12 1.12 1.12 1.12 1.12 Material hardness (Shore D) 5353 53 53 53 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.645.6 45.3 45.4 45.3 Deflection C (mm) 2.9 3.2 2.9 3.3 3.7 Surfacehardness (Shore D) 61 61 61 61 61 Core surface hardness − −5 −7 −7 −9−12 Ball surface hardness (Shore D) Ball surface hardness − Intermediate−2 −2 −2 −2 −2 layer surface hardness (Shore D) Intermediate layerthickness − Cover thickness (mm) 0.9 0.9 0.9 0.9 0.9 Deflectiondifference (A − C) 1.0 1.1 0.8 0.9 1.1 Comparative Example 4 5 6 7Structure 3-piece 3-piece 3-piece 3-piece Core Diameter (mm) 37.7 37.737.7 37.7 Weight (g) 32.5 32.3 32.7 32.7 Deflection A (mm) 4.2 3.9 3.83.8 Hardness profile (JIS-C) Surface hardness (Cs) 82 85 82 82 Hardnessat position 73 76 72 72 15 mm from center (C15) Hardness at position 6062 69 69 10 mm from center (C10) Hardness at position 59 61 69 69 5 mmfrom center (C5) Center hardness (Cc) 56 57 61 61 Cs − C15 9 8 9 9 C15 −C10 13 14 3 3 C10 − C5 1 2 0 0 C5 − C0 3 3 8 8 Cs − C10 22 22 13 13 C10− Cc 4 5 8 8 (Cs − C10)/(C10 − Cc) 5.5 4.4 1.6 1.6 Surface − Center (Cs− Cc) 26 27 21 21 Surface hardness (Shore D) 54 56 54 54 tan δ at 0.1%strain 0.0400 0.0420 0.0440 0.0440 core center 1% strain 0.0440 0.04500.0460 0.0460 10% strain 0.0550 0.0580 0.1000 0.1000 tan δ slope for0.0012 0.0014 0.0060 0.0060 10% strain and 1% strain tan δ at 0.1%strain 0.0690 0.0730 0.0750 0.0750 core surface 1% strain 0.0720 0.07500.0790 0.0790 10% strain 0.1350 0.1320 0.1400 0.1400 tan δ slope for0.0070 0.0063 0.0068 0.0068 10% strain and 1% strain Difference in tan δslopes 0.0058 0.0049 0.0008 0.0008 Core Center (ppm) 1980 2020 992 992moisture Surface (ppm) 1827 1802 1795 1795 content Surface − Center(ppm) −153 −218 803 803 Initial Day 0 of standing (V0), m/s 77.45 77.6977.63 77.63 velocity of Day 60 of standing (V60), m/s 77.03 77.29 76.8876.88 core after Initial velocity 0.42 0.40 0.75 0.75 standingdifference (V0 − V60), m/s Core surface Number of scratches 245 220 11090 roughness Core and Peel value (N/4 mm) 1.18 1.12 0.74 0.59intermediate layer Intermediate Material I I I I layer Thickness (mm)1.7 1.0 1.7 1.7 Specific gravity 0.95 0.95 0.95 0.95 Material hardness(Shore D) 56 56 56 56 Intermediate Diameter (mm) 41.1 39.7 41.1 41.1layer-encased Weight (g) 40.4 36.8 40.6 40.6 sphere Deflection B (mm)3.5 3.5 3.3 3.3 Surface hardness (Shore D) 63 63 63 63 Intermediatelayer surface hardness − 9 7 9 9 Core surface hardness (Shore D)Deflection difference (A − B) 0.7 0.4 0.5 0.5 Cover Material III II IIII Thickness (mm) 0.8 1.5 0.8 0.8 Specific gravity 1.12 1.12 1.12 1.12Material hardness (Shore D) 56.5 53 53 53 Ball Diameter (mm) 42.7 42.742.7 42.7 Weight (g) 45.1 45.6 45.3 45.3 Deflection C (mm) 3.1 3.0 2.92.9 Surface hardness (Shore D) 64 60 61 61 Core surface hardness − −10−4 −7 −7 Ball surface hardness (Shore D) Ball surface hardness −Intermediate 1 −3 −2 −2 layer surface hardness (Shore D) Intermediatelayer thickness − Cover thickness (mm) 0.9 −0.5 0.9 0.9 Deflectiondifference (A − C) 1.1 0.9 0.8 0.8

In addition, the flight performance (W#1), spin performance on approachshots, feel, scuff resistance, and durability to cracking of the golfballs obtained in the respective Examples of the invention and theComparative Examples were evaluated according to the criteria indicatedbelow. The results are shown in Table 4.

Flight Performance on Shots with a Driver

A driver (W#1) was mounted on a golf swing robot, the distance traveledby the ball when struck at a head speed (HS) of 45 m/s was measured, andthe flight performance was rated according to the criteria shown below.The club used was a TourStage X-Drive 709 D430 driver (2013 model; loftangle, 9.5°) manufactured by Bridgestone Sports Co., Ltd. The above headspeed corresponds to the average head speed of mid- and high-levelamateur golfers.

Rating Criteria:

-   -   Good: Total distance was 233.0 m or more    -   NG: Total distance was less than 233.0 m

Spin Performance on Approach Shots

A sand wedge was mounted on a golf swing robot, and the spin rate of theball when hit at a head speed (HS) of 20 m/s was rated according to thefollowing criteria.

Rating Criteria:

-   -   Good: Spin rate was 5,700 rpm or more    -   Fair: Spin rate was at least 5,600 rpm but less than 5,700 rpm    -   NG: Spin rate was less than 5,600 rpm

Feel

Sensory evaluations were carried out when the balls were hit with adriver (W#1) by amateur golfers having head speeds of 40 to 50 m/s. Thefeel of the ball was rated according to the following criteria.

Rating Criteria:

-   -   Good: Six or more out of ten golfers rated the feel as good    -   Fair: Three to five out of ten golfers rated the feel as good    -   NG: Two or fewer out of ten golfers rated the feel as good

Here, a “good feel” refers to a feel at impact that is appropriatelysoft.

Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot and the ballwas hit once at a head speed of 35 m/s, following which the surfacestate of the ball was visually examined and rated as follows.

Rating Criteria:

-   -   Good: The ball was judged to be still capable of use.    -   NG: The ball was judged to be no longer capable of use.

Durability to Cracking

The same type of driver (W#1) as in the flight performance evaluationwas mounted on a golf swing robot and the ball was repeatedly struck ata head speed of 45 m/s. For the ball in each Example, a loss ofdurability was judged to have occurred when the initial velocity of theball fell to or below 97% of the average initial velocity for the firstten shots. The average value for three measured golf balls (N=3) wasused as the basis for evaluation in each Example. The durability indexesfor the balls in the respective Examples were calculated relative to anarbitrary index of 100 for the number of shots taken with the ball inExample 1, and the durability was rated according to the followingcriteria.

Rating Criteria:

-   -   Good: Durability index was 90 or more    -   Fair: Durability index was at least 80 but less than 90    -   NG: Durability index was less than 80

TABLE 4 Example Comparative Example 1 2 1 2 3 4 5 6 7 Flight W#1 Spinrate 2,788 2,728 2,878 2,813 2,698 2,671 2,938 2,878 2,878 HS, 45 m/s(rpm) Total 234.4 233.8 232.3 231.6 230.4 234.8 230.1 232.3 232.3distance (m) Rating good good NG NG NG good NG NG NG Performance Spinrate 5,824 5,724 5,788 5,729 5,669 5,588 5,879 5,788 5,788 on approach(rpm) shots Rating good good good good fair NG good good good FeelRating good good good good good good good good good Scuff Rating goodgood good good good good good good good resistance Durability Ratinggood good good good good good good fair NG to cracking

In Comparative Example 1, the hardness profile of the core fell outsidethe range in values for the invention. As a result, the spin rate roseon full shots with a driver (W#1) and a good distance was not achieved.

In Comparative Example 2, the hardness profile of the core fell outsidethe range in values for the invention. As a result, the spin rate roseon full shots with a driver and a good distance was not achieved.

In Comparative Example 3, the hardness profile of the core fell outsidethe range in values for the invention, making the core soft and holdingdown the spin rate on W#1 shots. As a result, the initial velocity ofthe ball when struck was low and a good distance was not achieved.

The ball in Comparative Example 4 had a cover that was harder than theintermediate layer. As a result, the spin rate of approach shots wasinsufficient, making the ball performance inferior in the short game.

The ball in Comparative Example 5 had a cover (outermost layer) that wasthicker than the intermediate layer. As a result, on W#1 shots, the spinrate rose and a good distance was not achieved.

In Comparative Example 6, the hardness profile of the core fell outsidethe range of values for the invention. As a result, the spin rate onfull shots with a driver (W#1) rose and a good distance was notachieved.

In Comparative Example 7, the hardness profile of the core fell outsidethe range of values for the invention. As a result, the spin rate onfull shots with a driver (W#1) rose and a good distance was notachieved. Also, the number of scratches on the core surface was smalland the durability to cracking was low.

Japanese Patent Application No. 2015-113941 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A multi-piece solid golf comprising a core, a cover and anintermediate layer therebetween, wherein a sphere comprising the coreand the intermediate layer which peripherally encases the core(intermediate layer-encased sphere) and the ball have respective surfacehardnesses, expressed in terms of Shore D hardness, which satisfy therelationship: surface hardness of ball≦surface hardness of intermediatelayer-encased sphere; the intermediate layer and the cover haverespective thicknesses which satisfy the relationship: coverthickness≦intermediate layer thickness; and the core has a hardnessprofile which, expressed in terms of JIS-C hardness, satisfiesconditions (1) to (6) below, wherein Cc is the JIS-C hardness at acenter of the core, C5 is the JIS-C hardness at a position 5 mm from thecore center, C10 is the JIS-C hardness at a position 10 mm from the corecenter, C15 is the JIS-C hardness at a position 15 mm from the corecenter, and Cs is the JIS-C hardness at a surface of the core:20≦Cs−Cc,  (1)0<C10−Cc≦10,  (2)C10−Cc<Cs−C10,  (3)15<Cs−C10,  (4)Cs≧80,  (5)andCc≧52.  (6)
 2. The golf ball of claim 1 which further satisfiescondition (3′) below:(Cs−C10)/(C10−Cc)≧3.  (3′)
 3. The golf ball of claim 1 which furthersatisfies condition (1′) below:26≦Cs−Cc.  (1′)
 4. The golf ball of claim 1 which further satisfiescondition (7) below:(C10−C5)≦(C5−C0)≦(Cs−C15)≦(C15−C10).  (7)
 5. The golf ball of claim 1,wherein the core is formed of a material molded under heat from a rubbercomposition comprising: (A) a base rubber, (B) an organic peroxide, and(C) water or a metal monocarboxylate or both.
 6. The golf ball of claim1 wherein, letting tan δ₁ be the loss tangent at a dynamic strain of 1%and tan δ₁₀ the loss tangent at a dynamic strain of 10% when the losstangents of the core center and the core surface are measured at atemperature of −12° C. and a frequency of 15 Hz, and defining the tan δslope as (tan δ₁₀−tan δ₁)/(10%−1%), the difference between the tan δslope at the core surface and the tan δ slope at the core center islarger than 0.002.
 7. The golf ball of claim 1 which satisfies thecondition V₀−V₆₀<0.7, where V₀ is the initial velocity of the core inthe golf ball after the intermediate layer and cover, collectivelyreferred to as “the core-covering layers,” have been molded, as measuredafter peeling away the core-covering layers, and V₆₀ the core initialvelocity measured 60 days after measuring V₀.
 8. The golf ball of claim1, wherein the intermediate layer is formed of a resin compositioncomprising: a combined amount of 100 parts by weight of the followingtwo base resins (I) and (II): (I) an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester terpolymer, or a metalneutralization product thereof, having a weight-average molecular weight(Mw) of at least 140,000, an acid content of 10 to 15 wt % and an estercontent of at least 15 wt %, and (II) an olefin-acrylic acid randomcopolymer, or a metal neutralization product thereof, having aweight-average molecular weight (Mw) of at least 140,000 and an acidcontent of 10 to 15 wt % blended in a weight ratio (I):(II) of from90:10 to 10:90; (III) from 1.0 to 2.5 parts by weight of a basicinorganic metal compound capable of neutralizing un-neutralized acidgroups in the resin composition; and (IV) from 1 to 100 parts by weightof an anionic surfactant having a molecular weight of from 140 to 1500,and wherein the component (I) and (II) resins each have a melt flow rateof 0.5 to 20 g/10 min, component (I) and component (II) have a melt flowrate difference therebetween of not more than 15 g/10 min, thecomposition comprising components (I) to (IV) has a melt flow rate of atleast 1.0 g/10 min, and a molded material obtained by molding thecomposition under applied heat has a Shore D hardness of 35 to
 60. 9.The golf ball of claim 1 wherein, when the core surface is photographedwith a camera and image data collected by the camera is image processedin such manner as to identify and digitize scratches appearing on thecore surface, the number of digitized scratches is 100 or more.